class_docstr: Add the `DefaultTokens.MASK_BEFORE` placeholder between
  the prompt and the response in an LM finetuning exemple.
  This is necessary to enable the 'zero-out prompt loss' mechanism.
code: def apply(self, example, is_train=False, stats=None, **kwargs):
      _src = " ".join(example["src"])
      if len(_src.split(self.response_pattern)) != 2:
          logger.info("The mask_before could not be inserted")
          return example
      prompt, response = _src.split(self.response_pattern)
      response = DefaultTokens.MASK_BEFORE.join(
          [self.response_pattern, response]
      )
      _src = "".join([prompt, response])
      example["src"] = _src.split(" ")
      example["tgt"] = _src.split(" ")
      return example
  
file_path: OpenNMT-py/onmt/transforms/insert_mask_before_placeholder.py
imports: from onmt.constants import DefaultTokens
  from onmt.transforms import register_transform
  from onmt.utils.logging import logger
  from .transform import Transform
label_desc: Use image node_id 4 for calling the InsertMaskBeforePlaceholdersTransform obj's underlying member method code with example usage: obj.apply(example, is_train, stats) and returns: example, example